1. Field
This application relates to time synchronization of data obtained from sensors distributed at different locations, specifically to synchronization of sensor data after they are timestamped with local clocks in a distributed sensor system.
2. Prior Art
In a distributed sensor system that has multiple sensor units at different physical locations, each sensor unit's local clock may be used to schedule sensor-data collection or to timestamp sensor-data samples, and synchronization and syntonization of the local clocks in the distributed sensor system are important for obtaining simultaneous data from the sensors. Clock synchronization sets the clocks to the same time at a given instant, and clock syntonization adjusts the clocks to the same clock frequency.
U.S. Pat. No. 5,894,450 (1999) to Schmidt et al. discloses a system that synchronizes the sensor data from a number of underwater vehicles by placing a synchronization subsystem in each underwater vehicle. The synchronization subsystem synchronizes the sensor data by periodically resynchronizing its clock with other synchronization subsystems' clocks. Since the minimum time interval required between resynchronization depends on the accuracy and stability of the clock in the synchronization subsystem, the synchronization subsystem typically includes a highly accurate clock to timestamp each sensor-data sample. Accurate clock synchronization is achieved by frequent optical or acoustic communication among the vehicles. Sensor data may be processed in each vehicle in real time or near real time, or they may be stored in a data unit and processed later, when the vehicle is docked at a network node or when the vehicle is recovered. Each vehicle requires an accurate clock in the synchronization subsystem and a wireless (optical or acoustic in this case) communication system for resynchronization of the clocks during data collection. Furthermore, the wireless communication system consumes additional electrical power for clock resynchronization. If the system is used for small battery-powered sensing devices, the use of a wireless communication system in each device increases the size and cost of the device because of the additional electrical components needed for the wireless communication system, and because of the extra energy drained from the battery, necessitating a larger battery.
U.S. Pat. No. 5,566,180 (1996) and U.S. Pat. No. 6,654,356 B1 (2003), all to Eidson et al., disclose a method for synchronization and syntonization of local clocks of two nodes in a data-communication network by having the nodes send local-time information to each other using the data-communication network, and then having each node compare the difference between the received local-time information and its own local-time information. When the purpose of the nodes is to perform control or sensing function, the synchronization method requires an accurate local clock in each node and a data-communication network for frequent synchronization and syntonization of the local clocks. The synchronization system is not suitable for small portable devices, because a wired data communication network would restrain the portability of the devices and a wireless data-communication network would increase the size, power consumption, and cost of the devices.
The need for time synchronization of sensor data of a battery-powered system is exemplified in a physical-activity monitoring system described by K. Zhang et al. in “Measurement of Human Daily Physical Activity”, Obesity Research, Vol. 11, No. 1, 2003, pages 33-40. The system uses five small sensors attached to the body of a human subject to monitor physical activity of the subject, with two sensors placed at the anterior sides of the thighs, two sensors at the inferior sides of the feet, and the fifth sensor below the angle of the sternum. The output electrical signal of each sensor is transmitted through a cable to a data-collection device worn at the waist, so that types of physical activity can be identified from the synchronous signals from all of the sensors on the body. For future work, the authors of the paper propose wireless transmission of the sensor signals during data collection to a data-collection device to alleviate the inconvenience of wearing multiple wired sensors. However, as discussed above, incorporating a wireless communication system in each sensor increases the size and cost of the sensor.
L. Bao and S. S. Intille describe a wireless physical-activity monitoring system in “Activity Recognition from User-Annotated Acceleration Data”, Proceedings of the Second International Conference on Pervasive Computing 2004, pages 1-17. In the system, accelerometers are placed on each human subject's right hip, dominant wrist, non-dominant upper arm, dominant ankle, and non-dominant thigh to recognize ambulation, posture, and other physical activities. During a data-collection session, data of the accelerometer at each location are timestamped with an independent quartz-crystal clock and stored locally, so that after data collection the accelerometer data from all the different locations can be processed together to identify the types of physical activity performed. To achieve synchronization of the data samples obtained with independent clocks without using wired or wireless communication links, all the accelerometers are shaken together simultaneously with a fixed sinusoidal pattern at the beginning and end of each data-collection session. Then, during data processing, the authors use a computer to visually align the peaks of these distinct beginning and end sinusoidal signal patterns among all the accelerometers. Finally, timestamps of acceleration data are linearly scaled between the manually aligned start and end points. The post data-collection synchronization technique described in this paper is not practical for widespread use, because it requires sinusoidal vibration of the accelerometers before and after each data-collection session and tedious manual alignment of the accelerometer data. For future research, the authors of this paper recommend using small wireless accelerometers and a mobile computer to receive the wireless accelerometer data.
Physical activity, as a major form of energy expenditure, is considered one of the most important factors in the etiology, prevention, and treatment of obesity, and the development of an accurate physical-activity monitoring system to estimate energy expenditure for both research and widespread use has become a pressing need. The pedometer, a conventional device for counting the number of physical steps a user takes, fails to monitor other types of physical activity, and alternative accelerometer-based devices that monitor more types of physical activity are often inconvenient or expensive—significant barriers for integration into mainstream society. Because simultaneous data from multiple sensors on different parts of the subject's body are required for identifying the types of physical activity performed, using a wireless communication port in each sensor to transmit data during data collection has been proposed by many leading researchers in the field as a way to overcome the inconvenience of wearing multiple wired sensors. However, this approach suffers from a number of disadvantages:
(a) A wireless data recorder worn by the subject or installed in a mobile computer nearby is needed to receive the simultaneous sensor data.
(b) The limited number of wireless signal channels available for public use can result in signal interference if users in close proximity wear similar devices.
(c) Intermittent signal loss may occur when metal objects in the vicinity absorb transmitted electromagnetic signals, or when an antenna orientation changes while a physical activity is being performed.
(d) The transmitted radio-frequency signals can be tracked and decoded by undesired third parties, compromising the privacy of the user.
(e) The required transmission power for wireless communication during data collection drains extra energy from the battery, necessitating a larger battery.
(f) The wireless components, such as external inductors and an antenna, increase the size of the sensor.
(g) The manufacturing cost is increased because of the wireless component cost and labor cost for testing and tuning the wireless communication ports.
The use of post data-collection synchronization instead of wireless communication during data collection for a physical-activity monitoring system retains the benefit of user convenience while avoiding the disadvantages of wireless communication discussed above. However, this approach has been unviable in the past because of the impracticality of existing techniques to compensate for the time drift of the independent quartz-crystal local clock located in each sensor unit. The accuracy of common low-cost surface-mounted quartz crystals is 0.002 to 0.010%, resulting in a time drift of 1.7 to 8.6 seconds a day. Previous attempts to synchronize accelerometer data have required shaking all the accelerometers of a multi-accelerometer system simultaneously with a fixed sinusoidal pattern before and after a data-collection session.